Direct-liquid fuel cells using, as a direct fuel, a liquid fuel such as methanol, ethanol, formic acid, 2-propanol and dimethyl ether have a simplified structure and the fuels are easy to handle. These fuel cells are thus expected to be applied to portable uses, mobile power sources and distributed power sources.
The direct-liquid fuel cells have a structure in which, for example, a polymer electrolyte membrane with proton conductivity is held by an anode (fuel electrode) and a cathode (air electrode). The anode is directly supplied with a liquid fuel, while the cathode is supplied with oxygen, whereby at the anode, the liquid fuel is oxidized and at the cathode, oxygen is reduced, so that electric energy can be collected to the outside.
However, in direct-liquid fuel cells, a problem is that the crossover of liquid fuel decreases potential at the cathode along with decrease in fuel utilization rate, which leads to considerable decrease in energy conversion efficiency of the cells. The liquid fuel crossover refers to a phenomenon where a liquid fuel permeates through a polymer electrolyte membrane and moves from the anode to the cathode. The liquid fuel that has reached the cathode is directly oxidized on the surface of a cathode catalyst, which decreases potential at the cathode.
As a cathode catalyst of the direct-liquid fuel cells, platinum catalysts or platinum alloy catalysts are generally employed. Platinum catalysts or platinum alloy catalysts have high activity and high stability, but exhibit high catalytic activity not just with respect to oxygen reduction reaction but also with respect to the oxidation reaction of the liquid fuels mentioned above, and thus promotes also the oxidation reaction of the liquid fuel that has reached the cathode as a result of its crossover. Consequently, an oxygen reduction potential at the cathode, forming a mixed potential together with a liquid fuel oxidation potential, is considerably decreased.
Direct-liquid fuel cells, in order to promote reaction at the anode and to suppress potential decrease at the cathode caused by fuel crossover, employ more amount of a platinum catalyst than fuel cells using hydrogen. However, since platinum is expensive and limited in its resource amount, the development of alternative electrode catalysts for direct-liquid fuel cells is strongly desired.
In order to suppress liquid fuel crossover in direct-liquid fuel cells, an electrolyte membrane causing less permeation of a liquid fuel or an electrolyte membrane causing no liquid fuel crossover have been developed (for example, see Patent Literatures 1 to 3).
However, in the electrolyte membranes described in Patent Literatures 1 to 3, significantly decreasing liquid fuel crossover while keeping high ion conductivity and stability is extremely difficult. Even if using an electrolyte membrane suppressing liquid fuel permeation to some extent, since liquid fuel is necessarily permeated to no small extent together with water permeation, potential decrease at the cathode cannot be avoided.
On the other hand, catalyst are reported which do not oxidize a liquid fuel that has reached the cathode as a result of its crossover, but selectively perform oxygen reduction only (for example, see Patent Literature 4 and Non-Patent Literatures 1 to 4).
However, the catalysts disclosed in Patent Literature 4 and Non-Patent Literatures 1 to 3 employ expensive noble metals such as palladium and iridium in large amount, and are thus economically disadvantageous. The catalyst disclosed in Non-Patent Literature 4, which does not use a noble metal and is thus inexpensive, does not provide an oxygen reducing ability sufficient as a catalyst for practical purpose.
Thus, the development of more inexpensive and high-performance electrode catalysts for a direct-liquid fuel cell is strongly demanded.
The catalyst disclosed in Patent Literature 5 employs an inexpensive zirconium (Zr)-based oxide; however, the oxygen reducing ability sufficient as a catalyst for practical purpose is not obtained.
Non-Patent Literature 5 reports that zirconium-based ZrOxNy compounds exhibit oxygen reducing ability.
Patent Literature 6 discloses, as a platinum-alternative material, an oxygen reducing electrode material containing a nitride of at least one element selected from Group IV, Group V and Group XIV elements of the long periodic table.
However, materials containing any of these non-metals have a problem in terms of their failure to achieve an oxygen reducing ability sufficient as a catalyst for practical purpose.
Patent Literature 7 discloses oxycarbonitrides obtained by mixing a carbide, an oxide and a nitride, and heating the mixture in vacuum, inert or non-oxidizing atmosphere at 500 to 1500° C.
However, the oxycarbonitrides disclosed in Patent Literature 7, which are thin film magnetic head ceramic substrate materials, are not studied from the viewpoint of their use as a catalyst.
Platinum, which is useful not only as a catalyst for the above fuel cell but also as a discharge gas treating catalyst or a catalyst for organic synthesis, is expensive and limited in its resource amount. Thus, the development of alternative catalysts also in these applications is strongly desired.